At the District, the majority of our tick surveillance focuses on three tick species, Ixodes pacificus, (western black-legged tick), Dermacentor varabilis, (American dog tick) and Dermacentor occidentalis, (Pacific coast tick). These ticks quest for hosts in the vegetation along trails, and are easily picked up by hikers and dogs. While these three species present the greatest risk of disease transmission to humans and pets, there are many more species of ticks that live in San Mateo County that we do not collect as part of our usual surveillance.

This year, in addition to testing for Lyme disease and hard-tick relapsing fever, the District conducted testing for Anaplasma phagocytophilum which causes the disease human granulocytic anaplasmosis (HGA). HGA is a tick-borne disease that causes reoccurring bouts of mild to moderate fever, aches, nausea and vomiting. It is carried by adults and nymphs of the western black-legged tick (Ixodes pacificus). The District conducted real-time PCR testing of adult ticks collected from parks in San Mateo County for the 2017-2018 water year (October through September).

The seasonal invasive Aedes surveillance primarily targets the detection of two mosquito species that have been expanding their range in California, Aedes aegypti, and Aedes albopictus. The presence of these mosquitoes is highly undesirable because they are aggressive human biters and also transmit diseases including dengue, chikungunya, yellow fever and Zika. From 2013-2015, the District supressed Ae. aegypti in a small area of Menlo Park, but no invasive Aedes have been detected since August 2015.

This summer, the laboratory has been assisted by Alexander Flores, who conducted most of the surveillance for invasive Aedes mosquitoes this season. For the past five years, Alex has worked for the District as a seasonal technician in the Operations department. This year he took a position in the Laboratory department for a new experience. Alex started the summer as the invasive Aedes technician, funded by a grant from Public Health Foundations Enterprises, Inc. on behalf of the California Department of Public Health.

San Mateo County

As of September 28, 2018, there have been 330 dead birds reported in San Mateo County. Of those, 103 have been suitable for testing and five have tested positive (5%) for West Nile virus (WNV). Two mosquito pools have tested positive for WNV, both from East Palo Alto during the month of August. Positive birds and pools for 2018 are summarized in the tables below. No sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

Every year the District collects ticks from recreational areas in the county to assess the risk of tick-borne disease. One way to measure this is by determining the minimum infection prevalence (MIP), which estimates what percentage of these ticks are expected to be carrying a given disease agent in a population. The laboratory has completed testing for two vector-borne diseases from ticks collected from November 2017 to May 2018.

Autumn is the time of year for termite swarms as adult winged termites (called alates), leave their colonies, mate, and establish new colony sites. Residents often see the swarms around their neighborhood or find discarded wings and dead termites scattered on the ground or caught in spider webs. Only a very small proportion of termites are successful in establishing a new colony.

As of August 31, 2018, there have been 291 dead birds reported in San Mateo County. Of those, 91 have been suitable for testing and five have tested positive (5%) for West Nile virus (WNV). Additionally, two mosquito pools have tested positive for WNV, both from East Palo Alto. Positive birds and pools for 2018 are summarized in the tables below. No sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018, although a sentinel chicken in Santa Clara County, near the border with San Mateo County, tested positive in August.

The vast majority of mosquito control work at the District targets mosquitoes when they’re in the aquatic larval stage. However, occasionally the mosquitoes need to be controlled in their adult stage, such as when there is a human health risk from West Nile Virus. At this District, adult mosquitoes are treated with a backpack in small, isolated areas or by truck for larger sections of a neighborhood.

San Mateo County

As of August 1, 2018, there have been 247 dead birds reported in San Mateo County. Of those, 66 have been suitable for testing and two have tested positive (3%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

As of June 29, 2018, there have been 176 dead birds reported in San Mateo County. Of those, 52 have been suitable for testing and two have tested positive (4%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

District laboratory staff conducted a rodent survey for hantavirus with California Department of Public Health (CDPH) staff on June 4-5 in La Honda. Peromyscus truei (pinyon mice) and Peromyscus californicus (parasitic mice) were trapped and blood samples were taken for testing. Kidney samples from any mice with positive blood samples will also be tested by the CDPH to be used for a study of the geographical distribution of different hantavirus strains in California.

During the summer months, the District receives numerous calls regarding yellowjackets and paper wasps. These social wasps both have yellow and black markings and are common in San Mateo County, but they have important behavioral differences.

It’s summer and that means peak season for the dog tick and Pacific coast tick (collectively called Dermacentor ticks)! These large, brown-colored ticks are very prevalent this time of year along the edge of trails and in brushy areas with tall grass all throughout California. Unfed, they can be nearly three times as large as the black-legged ticks that are prevalent during the winter and spring and can engorge to the size of a grape if left attached to repletion.

As of May 25, 2018, there have been 115 dead birds reported in San Mateo County. Of those, 25 have been suitable for testing and two have tested positive (8%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

The laboratory began yearly surveillance for hantavirus with a rodent survey at San Bruno Mountain on May 2-3. Peromyscus maniculatus (deer mice) were trapped and blood samples were taken for testing. The mouse carcasses will be tested by the California Department of Public Health (CDPH) and positive samples will be used for a study of the geographical distribution of different hantavirus strains in California.

Laboratory staff continued collecting ticks in the nymph life stage during the month of May. Collections in May occurred at Laurelwood Park in San Mateo, Wunderlich County Park in Woodside, Thornewood Open Space Preserve near Woodside, Coal Creek Open Space Preserve near Portola Valley and Water Dog Lake Park in Belmont. Ixodes pacificus nymphs are active in the spring, and more difficult to collect in abundant numbers than adult ticks because they are typically on fallen logs or in leaf litter, whereas adult ticks are easily found on the edges of vegetation along trails.

During May, the District received several calls and requests for service regarding rat mites. Rat mites are ectoparasites that live in rodent nests and feed on their blood. These mites require rat blood to survive, but if rats have been recently eradicated from a home or the infestation is large, they will also bite people out of desperation. The most common rat mite San Mateo County is Ornithonyssus bacoti, the tropical rate mite, but there are several other species.